Acoustic filter for telephone receivers



1954 A. G. BODINE, JR

ACOUSTIC FILTER FOR TELEPHONE RECEIVERS Filed March 18, 1952 4 INVEN TOR. 4mm?" 61 flap/Nah ACOUSTIC FILTER FOR TELEPHONE RECEIVERS Albert G. Bodine, Jr., Van Nuys, Calif.

Application March 18, 1952, Serial No. 277,208

1 Claim. (Cl. 179-187) This invention relates generally to telephone receivers, and to acoustic filters for such receivers by which high frequency background noise can be filtered out without interference with the higher speech frequencies. As stated, the invention is particularly applicable to telephone receivers, though in a broadened aspect, it should be regarded as applicable generally to acoustic filters and attenuators, useful in various other fields.

Telephone systems are notoriously weak in reproduction of the higher range of voice frequencies. The trouble is aggravated by the fact that electrical background noise is characteristically strong in the uppermost range of audible frequencies, with the consequence of masking the already weak relatively high voice frequencies. The present invention aims broadly at the filtering out of the high audio-frequency background noise above the normal speech frequency range, while leaving the speech frequencies below that range undisturbed. Indeed, the cut-off frequency may actually, and without severe penalty, be taken low enough to remove also some of the upper register speech frequencies or harmonics. Thus it may be set at about 3,000 C. P. S., though 4,000'C. P. S. may desirably improve its quality, without greatly increasing the background noise.

A primary object of the invention is the provision of a simple acoustic filter, preferably a simple attachment to a conventional telephone receiver, by which objectionable high frequency background noise, together with unimportant high speech frequencies, can be filtered out without material interference with the important speech frequency range.

Another object is the provision of such a device which has the further feature that it is sufiiciently uncoupled from the electrical transmission lines to avoid reacting thereon.

A still further and more general object is the provision of a novel and improved form of acoustic low pass filter, or high frequency attenuator.

In the drawings, showing two illustrative embodiments of the invention:

Figure 1 is a medially-sectioned view of a telephone receiver embodying one form of the invention;

Figure 2 is an enlarged diagram showing the profile of the horn passage as viewed in a radial plane;

Figure 3 is a diagram showing the form of the crosssectional area of the horn passage; and

Figure 4 is a view similar to Figure 1 but showing a modification.

In Figure 1 of the drawing, numeral 10 designates a fragmentarily illustrated telephone instrument, having handle 11, and receiver 12 at one end of the latter. The electromagnetic sound wave reproducer unit 13, which may be of any conventional type, is mounted in the usual receiver bowl 14, integrally formed with handle 11. Bowl 14 has an annular flange 15, internally recessed, as at 16, to receive the receiver unit 13, and externally threaded, as at 17, to take the internally threaded flange 18a of receiver cap 18. This cap 18 is dished on its outside surface 19, forming a thin central section which is provided with perfortaions 20 for passage of sound waves from the reproducer unit.

The parts so far described are illustrative of present standard practice in telephone receivers.

According to the present illustrative embodiment of the invention, an auxiliary acoustic filter cap 24 is mounted on receiver cap 18. For this purpose, the periphery of receiver cap 18 may be reduced and screw-threaded for a United States Patent I 2,697,761 Patented Dec. 21, 1954 distance back from its forward face, as at 26, and the cap 24 is provided with an internally threaded flange 27 adapted to screw onto the cap 18, as indicated. The central area of the front wall 29 of cap 24 has perforations 28 for passage of the filtered sound wave.

The inside surface 30 of cap wall 29, opposed to the exterior surface 31 of the cap 18, is contoured so as to form with said surface 31 a thin annular chamber 32, which has an annular opening or mouth 33 adjacent the aperture 28, and which is thickest at mouth 33, and converges in thickness in a radially outward direction to substantially or nearly zero clearance at the periphery of the chamber. This annular chamber 32 is actually a radial sound wave passage, wherein sound waves enter through the annular opening or mouth 33, travel radially outward between the surfaces 30 and 31, and are destroyed as sound waves in the outer regions where the wall surfaces 30 and 31 become very close spaced. This radial sound wave passage 32 is dimensioned and contoured to be, in effect, an exponential horn, or equivalent.

An exponential horn is one whose cross-sectional area is halved at equal increments of length from mouth to throat. Applying this law to the present device, the crosssectional area of the annular, radially extending horn passage is halved for successive equal increments of length measured along radiating lines extending from horn mouth to outer periphery.

It will be seen that, for radially traveling sound waves, that is to say, waves entering at mouth 33 and traveling radially outward toward the constricted periphery, the cross-sectional form of the passage is a cylinder (see Figure 3) varying in area from a maximum at mouth 33 to a minimum at the constricted periphery, following the usual formula for an exponential horn. For any given station, between mouth and periphery, this cylinder will evidently have an area equal to 21111, where r is the radius, i. e., distance of the given station from the central longitudinal axis A-A, and t is the thickness of the passage 32 at that radius.

Since the dimension r constantly increases in the radially outward direction, the major dimension 21rr increases and therefore the minor dimension I must diminish very rapidly in order for the cross-sectional area of the horn passage (product of major and minor dimensions) to conform to the exponential horn function. This means that the wall surfaces 30 and 31 approach one another very rapidly, and there is a substantial area wherein said surfaces are very closely spaced. This kind of horn, increasing in one lateral dimension while decreasing in the other, results in an ultimate of practicability because of compactness. It replaces a very long attenuator horn of conventional shape. Moreover, this horn gives a large amount of attenuative wall area in the region where attenuation occurs.

From the familiar theory of exponential horns, it is known that the greater the rate of taper, the higher is the cut-off frequency. For present purposes, a cut-off frequency high in the speech frequency range is desired, for example, at about 4,000 cycles. Hence, the horn must have a relatively high taper ratio. For example, the crosssectional area may be halved at something between the approximate limits of A" and Assuming further the thickness t of the passage at the mouth of the horn to be about /s, and a radius r at the mouth of A", the two surfaces 30 and 31 approach one another to a clearance dimension t of approximately .010 within a half-inch of the mouth. Outside this radius, the spacing of the surfaces 30 and 31 of course progressively diminishes at the same fast rate, giving a large area throughout which the spacing distance t of the wall surfaces is less than .010". Sound wave travel through such a restricted passage is highly dissipative of energy, as will be further referred to hereinafter.

In operation, sound waves emitted through perforations 20 travel past the horn mouth 33 prior to being emitted from aperture 28. Waves of frequency below the cutoff frequency for the horn, e. g., 4,000 cycles, are rejected by the horn. Waves above the cut-off frequency are accepted by the horn, however, and travel radially outward therein without substantial reflection back. When these waves reach the region where the side surfaces 30 and 31 of the horn are relatively close spaced, for instance, by a spacing distance of .01-, or less, they are rapidly attenuated. This attenuation comes about first by reason of crowding of the waves, resulting in energy dissipation by frictional scrubbing of the oscillating gas particles on one another and onthe extensive defining surfaces30 and 3t of the passages. Second, crowding of the waves in the narrow passage'produces a steep wave front, resulting in conversion to higher frequency components, which process is dissipative of sound wave energy. The higher audiofrequencies, above the cut-off frequency of the horn, are thus received by the horn, and dissipated by the, constricted throat region thereof. The speech frequencies belowthe cut-off frequency are rejected by the horn, and freely passed through the aperture 28.

The device as thus described is suficiently coupled to the sound wave emitting aperture 20 of the receiver to capture and attenuate a large share of the waves above the cut-off frequency. The device is, however, not sufficiently coupled to the reproducer unit 13 to react on the telephone transmission lines.

Figure 4 shows a modification ofthe receiver of Figure 1. For convenience, parts in Figure 4 corresponding to parts in Figure 1 will be identified with like reference numerals, but with the sufiix a added, and precisely corresponding parts will not be again described.

In the device of Figure 4, the horn mouth 33a is about double the width of that in Figure 1; and the horn defining surfaces 30a and 31a are contoured to follow the exponential law only until the spacing distance has been reduced to something of the order of Outside of the circle at which the surfaces 30a and 31a have approached one another to this spacing distance, they are made parallel to one another. Packed within the clearancespace so provided is a pad 40 of some good sound wave absorbing material, preferably fiber glass.

Operation with this embodiment is similar to that with the form of Figure 1. The attenuative effect here, however, is provided by the fibrous material at 40. The gas particles of the sound wave coming into frictional impact with the fibers of this material rapidly lose their velocity energy, and the sound wave is thus dissipated by conversion to heat.

The device as thus described will be-seen to have other possible applications. It may be used in many situations, entirely separate from a telephone receiver, as a wave. attenuator, or as a low pass acoustic filter.

Itis my belief, however, that one of the greatest utilities of the invention arises out of my discovery, as aforementioned, of the improved intelligibility of telephone speech reproduction (which is notoriously poor in the high frequencies) obtained by the simple step of atteuation of the high end of the emitted sound. This step is of course opposite to the one that seems most natural, i. e., building up the weak high end, which is being masked by high frequency noise. But by attenuating the high end, the highfrequency noise is removed, and thatadvantage is much more than suflicient to compensate for a sacrifice of some of the high end; speech frequencies. The unexpected over-all result is general improvement in intelligibility. My device for carrying this filtering step into practical effect is of particular advantage because of its lack of coupling to the telephone receiver,'which results in filtering of the emitted sound without reacting back on the telephone lines.

The drawings and description will be understood asdisclosing merely present illustrative embodiments of the invention, and it will further be understood that various changes in design, structure and arrangement may be made without departing from the spirit and scope of the invention asdefined in the appended claim.

I claim:

For use with a telephone receiver cap having a reproducer unit including a diaphragm, and a centrally perforated receiver cap, a centrally perforated filter cap peripherally attached to said receiver cap whereby said receiver cap is disposed between said reproducer unit and said filter cap, said filter cap having an inner wall surface exponentially profiled to. define, with the outer wall surface of the receiver cap, a radially outwardly extending sound wave filter passage converging exponentially in the radially outward direction, said exponential sound wave filter passage being spaced from said reproducer unit by said receiver cap whereby said passage is acoustically decoupled from said reproducer unit andemitted sound is filtered without reacting back on said reproducer unit.

References Cited in the file of this: patent N D S AT PAT N Number Name Date 1,204,136 Creveling Nov. 7, 1 916 1,778,400 Pocock et al Oct. 14-, 1930 2,110,908- Hartmann Mar. 15, 1 938 2,205,670 Pye June 25, 1940 2,363,175 Grossman Nov. 2, 1 944 

